Aspirators, which provide a gaseous mixture and air, and humidifiers or nebulizers, which provide moist oxygen/air mixtures containing liquid droplets, are commonly used in inhalation therapy. Nebulizers can be constructed so that the oxygen/air mixtures they produce contain droplets of controlled size, which can be used to carry medicament to a patient's lungs. A single apparatus can function as both an aspirator and a nebulizer; when an aspirator is used, no liquid is supplied to the apparatus, whereas when a nebulizer is required liquid is supplied.
Aspirators and nebulizers are conventionally designed so that the energy needed to supply oxygen/air mixtures under positive pressure to patients, and in the case of nebulizers to nebulize the liquid, comes from a pressurized source of oxygen. Oxygen from this source is diluted with air and (if necessary) laden with medicament in the nebulizer in such proportions as prescribed by a health care provider.
The requirements for aspirators and nebulizers for use in inhalation therapy are becoming increasingly stringent since, in the long-term treatment of respiratory patients with moist oxygen/air mixtures, there is a trend towards the use of such mixtures with lower oxygen concentrations. Whereas oxygen/air mixtures containing 35% oxygen were typically used in the recent past, mixtures containing less than 30% oxygen are now desired. As illustrated in FIG. 4 below, the amount of air required to dilute pure oxygen (the only concentrated form of oxygen normally available in health care facilities) to produce a final concentration of oxygen in the oxygen/air mixture below, say, 30% oxygen is quite large. For example, to dilute a 29% oxygen mixture down to 27% oxygen requires an increase in the amount of diluting air of about 30%. Most current aspirators and nebulizers are not capable of diluting an oxygen stream to below 30% oxygen at moderate flow rates because of the large volume of air required.
However, although mixtures containing less than 30% oxygen are preferred for long term treatment, short term treatment sometimes requires undiluted oxygen. For example, a patient recovering from surgery may require a high-oxygen mixture (or even 100% oxygen) to flush remaining anesthetic from the patient's body. (For convenience, the term "oxygen/air mixture" is used herein to include the extreme case in which the "mixture" comprises 100% oxygen.)
At the same time, it is desirable that the aspirator or nebulizer be capable of producing a total oxygen/air mixture flow rate of at least about 40 liters/minute at all available oxygen concentrations in the mixture. Normal breathing requires a considerably lower rate of gas flow, but a traumatic event may temporarily require a flow rate as high as 40 liters/minute. In the high-dilution situation (i.e. when the oxygen/air mixture contains approximately 30% oxygen) an oxygen flow rate of less than 15 liters/minute is sufficient to yield a 40 liters/minute oxygen/air mixture flow rate. At low or zero dilution (i.e. when the oxygen content in the oxygen/air mixture approaches or reaches 100%) the rate of flow of oxygen alone must of course attain 40 liters/minute. Conventional aspirators and nebulizers are not able to achieve an oxygen flow rate as high as 40 liters/minute in a single unit at such high concentrations of oxygen (although they can be linked together in parallel to achieve this high, oxygen flow rate).
Finally, aspirators and nebulizers should be designed so that the oxygen concentration in the oxygen/air mixture remains fairly constant even though the flow demand may change, and should also be designed so that they can be manufactured sufficiently cheaply to be disposable.
As indicated above, no prior art aspirator or nebulizer meets all the foregoing requirements, and accordingly this invention is to provide an aspirator (also usable as a nebulizer) which can meet all these demands.